1. Field of the Invention
The present invention relates to a polarization independent optical isolator and, more particularly, to an optical isolator which requires only one birefringent optical device.
2. Description of the Prior Art
Optical isolators find a wide variety of applications in lightwave communication systems. In general, isolators are utilized to prevent reflected portions of a transmitted signal from re-entering the transmitting device. Many early isolator designs use a polarization-selective device to remove the unwanted reflections. In certain circumstances, where the transmission system may cause unknown, uncontrollable changes in polarization so that the polarization state of the received signal is unknown, these early devices are not considered to be of practical use. Thus, a large effort has been undertaken to develop an isolator which is polarization independent.
One prior art polarization independent optical isolator is described in the article "Polarization-Independent Isolator for Fiber Optics" by T. Matsumoto, appearing in The Transactions of the IECE of Japan, Vol. E62, No. 7, July 1979, pp. 516-7. In the Matsumoto isolator, the arrangement consists of a Faraday rotator and compensator (half-wave plate) inserted between a pair of birefringent crystal plates of equal thickness. As is well-known in the art, a birefringent plate functions to split an incident optical signal into a pair of orthogonal rays. Additionally, a birefringent plate functions to physically separate one ray (referred to as the "extraordinary" or "E" ray) from the other ray (referred to as the "ordinary" or "O" ray) as they travel through the plate. This phenomenon of spatial displacement is often referred to as "walkoff". In the Matsumoto isolator, a signal entering the first birefringent plate is split into orthogonal components. The rays are then rotated as they pass through the compensator and Faraday rotator. The two rays then enter the second birefringent plate (of as close a physical match to the first plate as possible) where they are recombined to form the output signal. Since a Faraday rotator is a non-reciprocal device, any signal traveling in the reverse (isolation) direction through the isolator will be physically separated into orthogonally polarized signals as it passes through both birefringent plates. One problem associated with this arrangement is that the thicknesses of the birefringent plates must be essentially identical since any difference will affect the power level of the recombined signal.
In an alternative prior art design, the compensator of the Matsumoto arrangement is replaced with an additional birefringent plate of a specific thickness. This particular design is described in the article "Polarization Independent Isolator Using Spatial Walkoff Polarizers" by K. W. Chang et al., appearing in IEEE Photonics Technical Letters, Vol. 1, No. 3, March 1989, at pp. 68-70. In this arrangement, the isolator consists of a first birefringent plate of thickness .sqroot.2 L, a Faraday rotator, and a pair of birefringent plates of thickness L, all arranged in tandem. Since the thickness of these last two plates is only 1/.sqroot.2 times that of the first, the two orthogonal rays will be recombined as they travel in the forward direction. Again, since the Faraday rotator is a non-reciprocal device, the rays will be further separated in the return direction as they pass through the first birefringent plate. However, as with the Matsumoto design, the ability to accurately control the thickness of the birefringent plates is crucial to achieving low loss in the forward direction and adequate isolation in the reverse direction.
Therefore, a need remains in the prior art for a polarization independent optical isolator which is less sensitive to the physical dimensions of the optical elements.